Sacral rami

Introduction

Reactive changes to the multifidus, ESA, gluteus maximus, SIJ ligs and TLF may narrow gaps that the dorsal sacral rami travel through (Steinke et al 2022). Similar changes may effect the dorsolateral sacral plexus (DLSP), that is formed from the anastomsis of the lateral dorsal sacral rami and their communicating branches, as it passes under (Roberts 2018) or through (Steinke et al 2022) the LPSL and descends to blend with the SCJ connective tissue (Fortin & Dill 2021). Sacral nerves branching off from the DLSP may also be afflicted from this soft tissue tightening as they pass through the LPSL, gluteus maximus aponeurosis (Konno et al 2017) and the TLF covering the STL (Willard et al 1998) to innervate the skin over the buttock. This can all account for sacroiliac and pseudosciatic pain (Willard et al 1998, Saunders et al 2018, Steinke et al 2022 & Anderson et al 2022).

The biceps femoris, gluteus maximus > piriformis stretches the STL (Vleeming et al 1989) to restrict sacral nutation (Vleeming et al 1991) due to its anatomical connections and by posteriorly rotating the innominate (Kuszewski et al 2018). As the STL is pre-strained by the two combined twists in its fibers, so that the medial IT fibers run superiorly and the lateral IT fibers run inferiorly, the lateral part of the biceps femoris, that attaches onto the lateral IT fibers, transmits tension horizontally towards the lower sacrum (Wingerden et al 1993) whilst the piriformis and gluteus maximus, attaching onto to the medial IT fibers (Vleeming et al 1989), transmit tension cephalically to the mid-upper sacrum and PSIS.

Also, the gluteus maximus, as it covers the IT, sends fascial expansions to the lateral and medial aspects of the IT to form a retinaculum. This retinaculum receives contributions from the biceps femoris to anchor the STL and hamstrings (< long head of biceps femoris). Therefore the gluteus maximus establishes a synergy with the biceps femoris through this retinaculum (Perez-Bellmunt et al 2015), with the obturator internus/urogenital diaphragm and the levator ani through fascial septa (Siess et al 2023) and with the thoracolumbar fascia and erector spinae aponeurosis through shared attachment sites (Willard et al 2012). This synergistic distribution of load between the gluteus maximus and these different structures determines mobility of the SIJ and its ligaments.

The STL can also be tightened by the gluteus maximus pulling the sacrum obliquely downward in the direction of the STL (Dontigny 2011) drawing the sacrum into the gluteus maximus (Abd-Eltawab et al 2023), and the piriformis exerting an oblique force on the sacrum pulling it down towards the lower pole of the SIJ (Mitchell et al 1979), creates a vertical shear of the sacrum at the SIJ (Pel et al 2008). As weight-bearing muscles the gluteus maximus and piriformis creating this downward vertical shear of the sacrum (Pel et al 2008) and posteriorly rotating the pelvis, also, along with its function as a hip abductor, tilts the pelvis down as bodyweight is transferred during load-bearing (Söztanacı et al 2021). This inferior translation of the sacrum (or superior translation of the innominate) (Hammer et al 2019) creates a vertical shear at the SIJ that is limited by the STL (Pel et al 2008), and again by the piriformis and gluteus maximus as their horizontal fibers, as well as their connections to the obturator internus/urogenital diaphragm and levator ani (Siess et al 2023), induces a force closure of the SIJ (Pool-Goudzwaard et al 1998). This stretching of the STL, including its medial IT fibers that run superiorly to the lateral side of the LPSL, along with any sacral counternutation (or diminished nutation) tightens the LPSL, especially when additional tension is applied from the erector spinae (Vleeming et al 1996). Stretching this LPSL-STL complex (including the STL’s attachment to the SCJ connective tissue (Kallini et al 2011) potentially exposes the lateral branches of the dorsal sacral rami, their communicating branches, the DLSP and its sacral branches to pathology (Konno et al 2017).

On the anterior side, the piriformis (especially when accessory fibers cover the sacral foramina), and its fascia which ensheathes the nerves, may compromise the ventral roots of the sacral rami which may account for SIJ/gluteal pain and distally referred or autonomic/visceral symptoms (Larionov et al 2022).

If chronic paraneural myo-fascial-ligamentous tightness around the lateral branches of the dorsal sacral rami results in neuroinflammation and nociceptive potentiation and peripheral and central sensitisation of these nerves (Macionis 2023) could it be the paraneural soft tissue ‘needling’ effect from sham RF on the sacral rami producing the same degree of pain relief as actual RF? (van Tilburg et al 2016). This paraneural ‘soft tissue release’ around the branches of the S2-3 lateral dorsal rami, that are most likely to pass through and innervate (McGrath & Zhang) a hypertrophied LPSL (Todorov et al 2022), become entrapped by the fascial roof over the STL, and if innervated, is most likely to innervate the posterior SIJ (Steinke et al 2022), would benefit diverse symptomology due to the range of different configurations in which these S2-3 nerves combine (Fortin & Dill 2021).

Dorsal Rami

The L5 lateral dorsal rami has a communicating branch that passes through the LPSL (Steinke et al 2022) as it descends to the S1 lateral dorsal rami. The passing of this communicating branch through the LPSL has not been noted by other authors although Roberts (2018) found L5 branches (but <S1-3), on their way to the DLSP, descending to the medial boarder of the LPSL. The L5-S1 lateral dorsal rami with its communicating branch, and possibly some S2 branches, innervate the interosseous SIJ ligs (Steinke et al 2022). Therefore, injury to the axial interosseous SIJ lig, a relatively weak and small part of the interosseous lig (Bechtel 2001), may account for L5/S1 (and possibly S2) referred sciatic pain (Steinke et al 2022 & Saunders et al 2018).

The syndesmotic part of the SIJ is posterior to the superior two thirds of the capsular part of the joint being composed of the fat-embedded interosseous ligs (Zou et al 2015). The interosseous ligaments stops the sacral base moving posteriorly and coordinates contraction of the muscles that laterally compress the pelvic ring to lock the sacrum into the pelvis (Saunders et al 2018) (‘force closure’) . It also provides a stable horizontal axis that distributes stress offering shock-absorbtion for the neurovascular structures that pass through (Poilliot et al 2023) its fat filled spaces to potentially innervate the (<superior) SIJ (Roberts 2018, Steinke et al 2022) .

The capsular part of the SIJ is anterior to the syndesmotic part being surrounded by a joint capsule that has an opening in its posterior part leading into the true synovial-lined, typically ‘L’ shaped, joint space. The irregular joint surfaces, along with the ‘wedging’ of the sacrum, ‘locks’ the SIJ into the pelvic ring (‘form closure’). These sacral and iliac irregular joint surfaces are diminished posteriorly by the ‘structural fat’ that surrounds it. This fat, acting as a cushion, deforms when it is compressed making the joint more rigid which pre-loads the ligaments the nerves run through and allows them to distribute stress more effectively (Poilliot et al 2019a). The superficial lamina of the STL, that is connected to the biceps femoris and gluteus maximus, not only covers the DLSP, and, by sending fan-like attachments to the joint capsule covers the posterior opening of the capsule that leads into the SIJ space, but, it is also continuous with the LPSL (Roberts 2018).

Branches from the lateral S2 and S3 dorsal rami travel through the fat filled spaces in the interosseous ligs, that is posterior to the superior two thirds of the capsular joint, towards the opening in the posterior SIJ capsule that leads into the SIJ space. However, as these nerves may not actually reach this gap the posterior SIJ may not be innervated (Steinke et al 2022). Other studies found the posterior SIJ was innervated directly by a superior lateral branch from the S1 lateral dorsal rami (40% of cases) and by nerves derived from the DLSP that pass through the interosseous lig to the superior (S1 sacral nerve) and middle and inferior parts of the joint (S1-S3 > L5 and S4 nerves) (Roberts 2018). In contrast, the anterior SIJ capsule definitively receives innervation from the L5 >L4&L5 >L4 ventral rami (Yilmaz et al 2023).

Branches from the S4 or S5 lateral sacral rami can pass through the proximal STL to innervate the skin over the medial region of the inferior gluteal region (Jiamjunyasiri et al 2023).

The S1-S4/5 lateral dorsal sacral rami, as they leave the sacral foramen, travel through tunnels composed of a continuous layer of loose connective and adipose tissue surrounded by a dense fibrous connective tissue. This aponeurotic tunnel originates from the medial aspect of each posterior sacral foramen and terminates by blending laterally with the ESA at the LPSL (McGrath et al 2010), or, continues from the posterior sacral foramen over the sacrum, deep to the ESA/TLF, LPSL and gluteus maximus aponeurosis to terminate over the ilum (Todorov et al 2022). It offers some protection to the lateral dorsal sacral rami as they wend their path either along the sacrum, or, variably through spaces between the laminae of the posterior SIJ lig (Dreyfuss et al 2008), ESA, and the TLF (Steinke et al 2022) attachments to the sacrum (Siccardi & Vale 2020), sacral erector spinae (Vleeming 2016), posterior SIJ lig (Todorov et al 2018) and LPSL (Steinke et al 2022). In contrast, the medial branches of the dorsal sacral rami exit the sacral foramen and then pierce this aponeurotic tunnel (McGrath et al 2010) to blend with, and innervate, the multifidus (Cox & Fortin 2014) and midline structures (Willard et al 1998).

The DLSP comprises of loops formed from the anastomosis of the lateral branches of the dorsal sacral rami (S1-3>L5/S4) and their communicating branches. It lies on top of the posterior surface of the sacrum and interosseous SIJ lig, and under the short posterior SIJ lig. With some fibers from the posterior SIJ lig and LPSL blending together (Poilliot et al 2019b), the lateral part of the DLSP passes either underneath (Roberts 2018) or through (Steinke et al 2022) the LPSL-superficial lamina of the STL complex. The DLSP originates adjacent to the S1-2 foramen, descends over the STL and then dives medially to terminate within the lateral and superficial posterior SCJ ligs and capsule (Fortin & Dill 2021). It gives rise to the lateral branches of the sacral nerve that form, amongst other nerves, the MCNs (Karl et al 2022).

As well as the DLSP, the MCNs (S1-4) also traverse the LPSL by passing through (<S2 & S3), over, or underneath it. The MCNs then pass through the gluteus maximus aponeurosis to innervate the skin over the buttock. The MCNs traverse the LPSL between (i) the PSIS, S1 MCN = pain 20mm caudal to the PSIS (Konno et al 2017) and (ii) the PIIS, S4 MCN = pain 41mm caudal to the PSIS (Konno et al 2017). At the PSIS and PIIS the LPSL, TLF, gluteus maximus, STL an ESA merge (Kallini et al 2019). The LPSL boarders are comprised of:

• Medially: the thicker and more stretched ESA/TLF (Todorov et al 2011) and multifidus (Vleeming et al 2012).

• Laterally: the thinner, looser gluteus maximus that overrides the ligament to blend with the TLF and posterior sacroiliac ligament (Rijnvos 1990) and attach onto the sacral spinous tubercles. The gluteus maximus also attaches on to the lateral part of the LPSL via the inferior gluteal aponeurosis (Todorov et al 2011). The STL’s medial IT fibers that cross to run superiorly to attach onto PSIS also form this lateral boarder (Vleeming et al 2012).

• Deep layer consisting of adipose and loose connective tissue the nerves (<S2 & S3) can traverse (McGrath et al 2009).

The medial-lateral boarders of the LPSL are blurred by the ESA/TLF and gluteus maximus aponeurosis (Todorov et al 2011). Therefore, just as stretching the gluteus maximus can reduce tension in the LPSL (Vleeming et al 1996) a thickening of the erector spinae or gluteus maximus aponeuroses can give SIJ pain via its LPSL attachments (Todorov et al 2022), especially when a branch of the MCN extends posteriorly towards the SIJ near the PSIS (Konno et al 2017). As well as anastomosis occurring between the MCN and the SCN and SGN (Konno et al 2017) a branch of the SCG enters the LPSL before heading towards, but not innervating, the SIJ (Cox & Fortin 2014) possibly accounting for lateral gluteal and referred lower extremity symptoms.

Having traversed the LPSL branches of the S1 (Willard et al 1998), S2 and S3 lateral dorsal sacral nerves (Steinke et al 2022, Willard et al 1998) then descends to another tunnel, the floor of which is formed by the STL, and the roof by a sheet of TLF that attaches cephalically to the LPSL (Willard et al 1998) and gluteus medius (Steinke et al 2022). Having pierced the fascial roof of this tunnel (Willard et al 1998) the nerves MAY reach the overlying gluteus maximus anterior surface as different lamellas of the STL and gluteal aponeurosis, that’s possibly the same anatomical structure, has gaps the nerves can run through (Steinke et al 2022) = buttock pain (Willard et al 1998).

Ventral Rami

The piriformis comes into contact with the ventral sacral rami as these nerves are sandwiched (i) posteriorly by the anterior surface of the piriformis; (ii) anteriorly by the parietal pelvic fascia (Williams and Warwick 1980) (including the piriformis fascia that ensheathes the nerves) and internal iliac vessels (Shafarenko et al 2022); (iii) laterally by the obturator internus (Gaertner 2006). Also, when present, accessory fibers of the piriformis cross the anterior sacral foramen (Sen & Rajesh 2011) < S2 where the S2 ventral rami provides somatic and autonomic fibers to the pelvis and leg (sciatic, pudendal, inferior and superior gluteal, femoral and posterior femoral cutaneous nerves) accounting for distally referred or autonomic/visceral symptoms (Larionov et al 2022). 

Pelvic preganglionic parasympathetic neurones arise from the S2-4 ventral sacral rami (>S1 and S5). They then pass through the parietal pelvic fascia (including the piriformis fascia) to form the pelvic splanchnic nerves. These nerves enter the inferior hypogastric plexus (Goidescu et al 2022), that is contained in the endopelvic fascia at S3 (Johnson 2012), to synapse with their postsynaptic neurones.

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